1. Field of the Invention
The present invention relates to a solid-state image sensor.
2. Description of the Related Art
CMOS image sensors are known as a type of solid-state image sensors. CMOS image sensors have features such as a single power supply, low-voltage driving, and low power consumption. Like CCDs, CMOS image sensors have been developed to have a greater number of pixels and a micro size, and to form photodiodes and transistors on the same substrate.
In CMOS image sensors, an electric potential of a part accumulating signal charges is modulated by signal charges generated by a photoelectric conversion element. Amplifier transistors inside pixels are modulated by the modulated electric potential, to perform an amplifier function inside the pixels.
Properly providing electrical isolation between photodiodes is very significant for CMOS image sensors to reduce signal noise. For example, if the photodiodes are constituted by N-type epitaxial layers and P-type diffusion layers. The electrical isolation is achieved by a P-type diffusion layer which surrounds another diffusion layer (for example, see Jpn. Pat. Appln. KOKAI Publication No. 2006-286933).
In some cases, however, signal noise occurs even with this electrical isolation provided.
Specifically, each of pixel transistors for reading signals from the photodiodes is constituted by a read transistor, a reset transistor, and an amplifier transistor, which are formed on a P-type diffusion layer.
When light is obliquely incident on the photodiodes in the configuration as described above, electrons are generated in a P-type diffusion layer (i.e., a neutral area of zero electric field) in some cases.
Electrons generated in the P-type diffusion layer move by diffusion to a different pixel transistor (for example, a detection portion of a read transistor) from a pixel transistor which originally should have detected the generated electrons, and become signal noise.